Apparatus for communication systems



' Sept. 18, 1945. w. L KONRAD APPARATUS FOR COMMUNICATION SYSTEMS Original Filed March 26, 1943 Y MV... 0a. E i A wp m NH n I? .A L am 1:@ m a Nvvw V .Sw ...a i w E. Il- I w n \Nw., M.m\ N @wm Sw MN i w .u\ .nl v N um ww a, mm s@ l@ @RW am. .TNN QN 1. ii A fr m .0 QN n |I 4 IINQ Q. mw Nw m. m I. i 1 4 .bwwx M Imm lum@ mm mm uw wN w N A. W Nm N wm. NN nwwsw@ wwkww KNNY QNNW SQSW mwxw NN Patented Sept. 18, 1945 APPARATUS Fon coruviUNIoA'rloN l SYSTEMS Y y William L. Konrad, Pittsburgh, Pa.,assignor to The Union Switch & Signal Companwswissvale, Pa., a corporation of Pennsylvania Original application March 26, 1943, Serial No. 480,645. Divided and this application March 27, 1944, Serial No. 528,228

1 claim. (01.250-20) carried signalingV device such as,` for example, a

loud speaker. Preferably, the receiving apparatus of such a train communication system remains Vnormally in the circuit readyto receive a message. This normal receiving condition results in amplification of minute earth currents, Astray magnetic elds and similar random energy during non-communication periods and noise may occur at the loud speaker. Also duringcommunication periods amplification of the noise energy may impair the'intelligibility of inf coming messages. Furthermore, the

energy level ofthe communication current at the re-' ceiving station may vary greatly due to the changing conditions of the transmitting channel through the rails and the changing distances between the station on a moving train and the remote station with which communication AisY being held, and as a consequence of the tubeis fed to the rectifying section which becomes conductive when the amplified Voltage overcomes the bias voltage. The energy thus rectified is applied to one or more of the earlier stage tubes of the receiving apparatus as a negative bias voltage to control the amplification gain. A unit consisting of a condenser and a resistor is included in the circuit of the automatic volume control means to form a time delay device Which maintains the control voltage between syllables and words of the received communication current.

For a better understanding of the invention, reference may be had to the accompanying drawing which is `a diagrammatic view showing one form of `apparatus embodying my invention when used with receiving apparatus for a lrailway'train communication system using a suppressed carrier single side band telephone current.Y It is to be understood, however, that the invention is not limited to railway train communicationsystems` and this one application serves to illustrate the many places where suchapparatus is useful.

Referring to the drawing, the receiving apparatus includes a receivinginductor, two stages such Variations of the energy level, an adjustment of the receiving apparatus is required if uniform level of reproduction atv the loud speaker is to be obtained;

Accordingly, an object of my invention is the provision of communication receiving Vapparatus incorporating improved automatic volume con-i trol means.

Other objects, features and advantages of myy invention will appear as the specification progresses.

I shall describe one form of apparatus embodying my invention, and shall then point out the novel features thereof in the claim.

According to my invention, I provide an automatic volume control means which preferably includes a twin triode electron tube.y A voltage of intermediate ,frequency amplification, a demo'dulator, a filter, one stage of audio frequency amplification and` a loud speaker. The reference character IC indicates a receiving inductor for picking up energy due to current sent out several well-known types and is shown as an in- 1Speciflca11y,inductor `IC is connected to pridueto the communication current is taken'from a preselected point in the receiving apparatus andfed through a resistance-capacitance lter to the control grid of one section of the twin,

triode tube to amplify such voltage. The other section of the twin triode tube is arranged as a rectifier to whose cathode a bias voltage' is applied. The output of the amplifying section of mary windingv 3j of transformer Tl through a condenser '4, .acondenser 5 is connected across primary winding 3, and secondary winding 6 of transformer TI is connected across control grid 1 and cathode 8 of `tube VI through two condensers 9 and l0 in series. Preferably, the junction terminal of condensers 9 and l0 is connected to a ground electrode I l and one terminal of primary lwinding'3 of transformer Tl is con- The plate circuit y for tube VI can be traced from positive termi-`V` nal B33!) of generator G through wire I3, resistor i4, plate I5 and intervening tube space to cathode 8 of tube VI, resistor I6 and ground electrodes I'I and I8 to negative terminal N300 of generator G. A first auxiliary grid I9 of tube Vi is connected to cathode 8 and the other auxiliary grid of the tube is supplied with positive voltage from generator G through resistor 2|.

It is clear that the electromotive force picked i up by inductor IC due to the communication current, is amplified in the plate circuit of tube Vl in the well-known manner. As an aid to the understanding of the operation of the-apparatus, I shall assume that thecommunication current is a suppressed carrier single side band telephone. current whose carrier frequency is of the orderv of A5700 cycles per second, the voice frequency band for modulating the carrier is of 400 to 2500 cycles per second, and the upper side band of 6100 to 8200 cycles per second is the side band selected for transmission. Thus the plate circuit current of tube -V I has a component corresponding to such communication current received through inductor IC.

The second stage of amplification includes an electron tube whose control grid circuit is coupled to the plate circuit of the first stage tube VI through a condenser -22. This second stage of amplification is'sho'wn conventionally for-the sake of simplicity because it can be of any wellknown construction and may be substantially a duplication of the first stage, and a description of the second stage is not required for Ya full understanding of my invention. It is sufiicient to point out that the communication current com` ponent passed from the plate circuit of tube VI to the control grid circuit of the second stage is amplified in the usual manner and is supplied from lthe output of Ithe second stage amplifier to the-input side of a demodulator DM through wires 23 and 2li.

Demodulator DM-may be any one of several well-known forms and is shown conventionally7 in order to not unduly complicate the drawing because its specific construction forms no part of my invention. For example, this demodulator may be of the balanced type covered by Letters Patent of the United `States No. 2,129,313, granted September 6, 1938 to L. D. Whitelock, for Apparatus for electric signaling systems. It is sufiicient for the present application to point out that in demodulator DM a carrier from a local oscillator OS is mixed with the side band telephone current supplied through the second stage amplifier, and the carrier is suppressed at a pushpull arrangement of primary winding 25 of a transformer T2 included in the output circuit of the demodulator. Thusy one product of demodulation to appear in primary winding 25 is the voice frequencies of the communication current received by inductor I C. f

Band pass filter BPF comprises transformer T2, condensers 26 and 21, inductance 28 and at least a portion of winding'29 of an autotransformer T3. Thisband pass filter isarranged in a Well-known manner'and is proportioned to pass fl. f

the voice frequencies and to substantially eliminate the other products of demodulation so that the voltages appearing across Winding 29 of transformer T3 are those of the voice frequencies, or at least of Aa frequency within the selected voice frequency range. l

The final and audio amplification stage of the receiving apparatus includes an indirectly heated pentode V5, whose filament is constantly heated and whose control grid 30 rand cathode 3| are l connected to winding 29 of the band pass filter through a grid circuit to appear shortly. Plate voltage for plate 32 of tube V5 is supplied from generator G through primary winding 33 of an output transformer T4 to whose secondary Winding 34 a loud speaker LS is connected. It followsl that energy passed by the band pass filter BPF is amplified by tube V5 and is applied to the loud speaker.

An automatic volume control unit AVC is provided for the receiving apparatus. This unit AVC includes a twin triode tube V'I, whose filaments are heated inthe usual manner. Voltage from a preselected terminal B of filter BPF is applied to control grid 41 and cathode 52 of the righthand triode of tube V1 'as viewed in the drawing, the circuit connection including wire 5I, a resistance-capacitance filter comprising resistor 8 and condensers 49 and 5U and a condenser 53. Plate voltage is supplied to plate 5G of the right-hand triode of tube V'I from generator G through wire I3 and resistor 54, the plate circuit being completed from cathode 52 through ground electrodes55 and I8 to negative terminal N300 of generator G. Thus the communication current voltage appearing at terminal B of the band pass lter is amplified by the righthand triode of tube VI of the automatic voliune control unit.

The left-hand triode of tube V'I is converted into a triode rectifier by its control grid 5l being connected to plate 58.A A bias voltage is applied to cathode 59 ofthe rectifier through resistors 60 and 6I connected across generator G and a resistor 62 connected between the cathode and a preselected mid terminal of resistor 60. Plate 58 of the rectifier is further connected to a ground electrode 55 through resistor 63.' Thus, the cathode 59 of the rectifier is madeA positive in potential with respect to the plate 58 by a voltage preselected by the connection to resistor 60. The amplied output of the triode of tube V'I is applied to the rectifier section of the tube through condenser E4 connected between plate 5E of the triode and cathode 59 of 'the rectifier. Consequently, whenthe voltage from terminal B after amplification at the triode of vtube V'I is greater than the bias voltage of the rectifier section of the tube, the voltage is rectified and current fiows in resistor E3, the top terminal as viewed in the drawing of resistor 63 being negative with respect to ground electrode `55.

This voltage created across resistor 63 is supplied to the control grid 'I ofthe first stage tube VI through a connection including wire 65 and resistor 66. Thus when the communication current is of a large amplitude and a relatively large voltage appears at terminal B of the band pass filter, the first stage tube VI is provided with an g nection including wire 68 extending from resistor 66 to the grid circuit of the second stage tube.

A condenser 61 is connected in shunt with resistor 63 to form a time delay device, condenser 6T being charged by the rectied voltage and such charge serving to maintain the automatic volume control voltage between syllables and words of the received communication current.

It is to be seen from the foregoing description that I have provided a communication receiving apparatus incorporating an automatic volume control unit which permits adjustment of the receiving apparatus for a relatively low energy level of communication current without excessive loudness when the communication current becomes of relatively high amplitude.

Although I have herein shown and described only one form of apparatus for communication systems embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

In an automatic volume control unit for carrier telephone current receiving apparatus having a first stage amplifier, a demodulator and a band pass filter adjusted to pass voice frequency energy of such telephone current; the combination comprising, an amplier tube having a plate circuit including a direct current source, an additional filter, a grid circuit including said additional iilter to connect a control grid of said amplifier tube to a preselected terminal of said band pass filter to create in said plate circuit an alternating voltage component directly proportional to the voice frequency energy of telephone current applied to said receiving apparatus, said additional iilter to substantially suppress carrier frequency energy appearing with said voice frequency energy to assure that said alternating voltage cornponent is substantially directly proportional to the voice frequency energy of the received telephone current, a triode rectifier having a circuit including a resistor and a source of bias voltage poled to bias the cathode of said rectifier a preselected positive voltage with respect to the plate of the rectifier, said amplier tube plate circuit coupled to said triode rectier circuit to create a rectied current through said resistor proportional to the magnitude of said alternating voltage component over said bias voltage, and circuit means to connect said resistor to a control grid of said rst stage amplifier to apply an automatic volume control voltage due to such rectified current. WILLIAM L. KONRAD. 

